The present invention relates to the transport of a continuous web of material and more particularly to a system and method for tracking of the web in its path of movement from a supply to a takeup means.
Many different kinds of systems have been devised to track the movement of a web of material in order to positively determine, for example, various locations along its length so that one or more operations may be performed in connection with the treatment of the web. In carrying out the treatment, the path of the web may have to be monitored to ensure that it .[.mantains.]. .Iadd.maintains .Iaddend.a predetermined path in the system for processing at one or more system stations. This may entail optical monitoring means and lateral translation of the web in the system path or lateral translation of the web supply roll to provide for misalignment correction.
Also, the web may change in physical size, i.e., it will stretch or expand, or shrink or contract both laterally and longitudinally relative to its length. Such expansion or shrinkage is due to several factors. The major factors are environmental conditions, e.g., temperature and humidity, web handling in the system and the resultant action of the particular processes being performed in connection with the web, e.g., the application of a fluid to the surface of the web.
In the usual case of web material, e.g., electrographic recording medium comprising dielectric coated paper, the web can stretch or shrink as much as 1 mil per foot and the dimensional change laterally across this type of material can be three times greater than the dimensional change along the longitudinal extent of the material. Web material is acceptable to such dimensional changes due to the manner by which it is made. For example, in the case of paper, the fiberous grain of the paper is such that it can stretch or shrink more in one orthogonal direction as compared to another. Web material such as polyester based films may not stretch or shrink as much as paper, but are still succeptable to some stretching and shrinkage.
Further, web material may neither be perfectly flat or straight nor are the web material edges exactly parallel to one another.
These web dimensional changes and physical irregularities which may occur while the web material is moving through a web processing system can contribute significantly to the successful application of the desired process.
While one solution to this problem might be to require tighter specifications in the design and manufacture of web material without these irregularities, this would not be desirable because of the high costs to provide such quality control in its manufacture, which would not be acceptable to web material manufacturers. The better approach is to create a tracking system that can cope with these irregularities and capable of monitoring and controlling the station functions without requiring changes to the web material.